Blog

Global Temperature Records - Old and New

21 Oct 2011, 10:50

Verity Payne

Earlier this week a Journal
of Geophysical Research paper detailed the latest updates to
the Global Historical Climatology Network (GHCN). The network
is a source of temperature measurements from around the globe that
is used to work out the average global temperature trend.

The release of the paper has coincided with the findings of the
Berkeley Earth Surface Temperature (BEST) study, made public
yesterday. So it seems the ideal time for a quick recap on how
average global temperature records are produced, and how the BEST
study differs:

How do scientists measure temperature at the Earth's
surface?

Systematic measurements in inhabited regions of the world began
in the
19th century, and have been complied into the GHCN. These days,
thermometer readings at meteorological stations, polar research
stations and on ships and buoys provide direct measurements of
surface temperature on land and at sea. Monthly updates are now
provided through the World Meteorological Organisation's
CLIMAT reporting system.

The temperature of the lower atmosphere can be determined from
satellite observations. These are not direct measurements - the
satellites monitor microwave radiation, from which they can infer
temperature.

Who produces global temperature trends?

Three main research groups produce records of average global
temperature using direct measurements at the earth's surface.

Two of the groups are based in the USA: the NOAA National
Climatic Data Center (NCDC),
and the NASA Goddard Institute for Space Studies (GISS). The
third, HadCRUT,
is produced by a collaboration between the UK Met Office Hadley
Centre and the Climatic Research Unit at the University of East
Anglia.

The Japanese Meteorological Agency (
JMA) also provides a global temperature record.

Two satellite records of lower atmospheric temperature come from
the University of Alabama in Hunstville (UAH)
and Remote Sensing Systems (RSS).
The two groups use the same satellite data with different methods
to determine temperature.

How do the three main groups compile global temperature
records?

Producing a global temperature record is not as straightforward
as just taking an average of all the measured temperatures over
time. This is because the stations are not evenly spread over the
globe, plus some stations have breaks in their measurements or have
poor data.

Stations can also be affected by non-climatic changes. For
example, it is well
established that built-up, urban areas can be warmer than the
surrounding countryside - the 'Urban Heat Island' (UHI) effect.

The three main research groups all use data from weather
stations around the globe, with the principal source for each group
being the GHCN, supplemented by additional sources. The precise
station make-up of each temperature record varies however according
to the different methodologies that they use.

The groups use different methods to ensure quality control of
the data, and to adjust any artificial bias from phenomena such as
the UHI effect. For example,
GISS identify urban and rural areas and compare temperatures
from neighbouring urban and rural areas to determine any UHI
effect.
HadCRUT simply assume UHI adds a slight warming error to their
temperature trend and add that on.

The datasets for the land surface are then combined with
temperature data for the sea surface. The researchers split the
surface of the globe into grid boxes and calculate a temperature
average for each grid box. The methods used by each research group
for these processes are different. See our Global Temperatures
profile for more details on these processes.

Do all of the different temperature records show the
same trend?

The different temperature records show very similar trends. This
is clearly demonstrated by the following graph, which compares the
GISS, NCDC, HadCRUT and JMA trends:

The two satellite records also provide a very similar trend to
the surface temperature trend, as shown in this graph:

Despite studies (here
and
here for example) showing that UHI has a minimal effect on the
overall temperature trend, climate skeptic commentators have
claimed that scientists have underestimated the UHI
effect, and that they don't take poorly sited stations into
account. The BEST
study set out to answer these concerns.

The BEST study uses a
different averaging methodology to the NCDC, GISS and HadCRUT.
BEST boast that their method can use temperature records that are
short and discontinuous, and it does away with the grid box
system.

Another difference is that NCDC, GISS and HadCRUT use the GHCN
as their main data set, supplemented by other sources. The GHCN
consists of average monthly temperatures from nearly 7300
meteorological stations, and requires that stations provide long,
reasonably complete temperature records. The three main compilers
use as much and as reliable data as possible.

For example,
HadCRUT currently uses data from around 4500 stations.
Their methodology rejects stations with less than 15 years of data
and instead uses temperature data from various regional climate
studies and archives in those regions.
GISS uses around 6300 meteorological stations, along with
Antarctic research stations.

However, their datasets are dwarfed by that of the BEST study,
which has gathered data from
nearly 40,000 unique stations (from 10 public
sources).

BEST have so far only
tried out their new averaging methodology on the NCDC dataset,
but hope to try their methodology on their full dataset in the
future. They have used
the full dataset to determine the effects of UHI on the global
temperature trend.

It should be noted that research has demonstrated
that reliable global trends can be compiled from a worldwide
network of fewer than 300 stations if the stations are well
maintained.

A further difference is that the BEST study has so far only
considered global land surface temperatures, and does not
incorporate the sea surface temperature trend.

What does the BEST study show?

Despite the differences in methodology, BEST study shows a
global temperature trend from land stations very similar to those
of NCDC, GISS and HadCRUT:

BEST researchers also tested a sample of 2000 stations that are
not part of the GHCN and found a very similar trend to those found
previously. The BEST study final report on UHI finds,
unsurprisingly, that its effect on the global land temperature
trend is minimal. We have more about the BEST UHI study here.

The BEST study publications have yet to be peer-reviewed. But
their findings are consistent with a wealth of scientific research
already conducted in this area. The big question now is whether
this will lay to rest the concerns of climate skeptics about the
reliability of global temperature records.